This invention relates to a process for the removal or reduction of dissolved hydrogen sulfide, and reduction of BOD in sewer systems, municipal waste treatment plants and in other industrial waste applications.
It is known to add nitrates or nitrites to sewage to effect reduction in BOD and even to suppress the formation of hydrogen sulfide gas via bacterial action. See, for example, U.S. Pat. Nos. 3,300,404; 4,446,031; and 4,681,687.
It is also known to add nitrates to sewage in order to control objectionable odors. See, for example, U.S. Pat. Nos. 3,966,450; 4,108,771.
There have also been attempts to remove hydrogen sulfide directly from waste. For example, in U.S. Pat. No. 4,680,127, the patentee adds amounts of glyoxal, or glyoxal in combination with formaldehyde or glutaraldehyde, in order to reduce or scavenge the amount of hydrogen sulfide in aqueous or wet gaseous mediums.
In U.S. Pat. No. 4,501,668, the patentee utilizes polycondensation products produced by the condensation of acrolein and formaldehyde to eliminate hydrogen sulfide present in aqueous systems, such as waste water clarification plants. Merk also mentions benefits relating to corrosion prevention and deodorization.
In U.S. Pat. No. 3,959,130, the patentee decontaminates sewage systems, waste water treatment plants and other industrial waste applications containing hydrogen sulfide by adjusting the pH of the sewage of a value over 7.0 and bringing the sewage into contact with an ash product.
It has now been discovered that the addition of nitrate, via an aqueous sodium nitrate solution, to sewage systems, waste treatment plants and other industrial waste applications containing dissolved hydrogen sulfide will result in the elimination or substantial reduction of the hydrogen sulfide, as well as the elimination of other "minor" odors associated with other sulphur-containing compounds.
It is believed that the addition of nitrate provides an oxygen source which promotes the growth of naturally occurring bacteria which utilize in their metabolism the sulfur tied up as hydrogen sulfide. It has been demonstrated both in lab jar tests and in an actual sewage collection system test, that dosing sewage containing over 50 mg/L of dissolved hydrogen sulfide with a sodium nitrate solution reduces the dissolved hydrogen sulfide to less than 0.1 mg/L. Along with this phenomena a significant reduction in sewage biological oxygen demand, BOD, of up to about 70%, and overall "sweetening", i.e., removal of other minor odors, of the sewage has been observed. These phenomena are believed to be the results of the biological process promoted by the nitrate addition.
More specifically, it has been found that 2.4 parts of nitrate oxygen (NO.sub.3 --O) are necessary to remove 1 part dissolved sulfide (S.sup.2-). The source of nitrate to accomplish removal of the hydrogen sulfide is not specific, and aqueous solutions of both sodium nitrate and calcium nitrate appear to be suitable.
Because the necessary reaction is biochemical, it will not occur within a sterile solution, i.e., naturally occurring bacteria must be present. Moreover, the removal of hydrogen sulfide is not instantaneous. According to applicant's tests, an "incubation" period of about 8 to about 96 hours, and preferably about 24 to about 48 hours, is necessary to culture the bacteria, followed by about 1.5 to about 20 hours, and preferably about 3 to about 12 hours, for ongoing sulfide removal.
It has further been determined that the process in accordance with this invention achieves a significant reduction in sewage BOD due to the utilization of organic matter in the metabolism described.
Other objects and advantages will become apparent from the detailed description which follows.